The present invention relates to wavelength tunable laser sources.
In the optical communication industry, there is a need for testing optical components and amplifiers with lasers continuously tunable over a wide wavelength range. So-called Littman cavities can be used as external cavities to allow single-mode tuning of the laser, as first described by Liu and Littman in “Novel geometry for single-mode scanning of tunable lasers”, Optical Society of America, 1981. The Littman cavity allows tuning wavelength and optical length of the cavity at the same time by changing only one parameter of the geometry, i.e. the tuning element. However, as already pointed out by Liu and Littman in the aforementioned document, mode hops might occur due to deviations of the real geometry with respect to the perfect Littman configuration. Fine adjustment e.g. of the laser end mirror is proposed to avoid mod hops.
Examples of tunable lasers, based on the Littman or other geometries, and various options for reducing or avoiding mode hops can be found e.g. in U.S. Pat. No. 5,867,512, DE-A-19509922, Wenz H. et al: “Continuously Tunable Diode Laser” in ‘Laser und Optoelekronik’ (Fachverlag GmbH, Stuttgart, D E, Vol. 28 No. 1, p. 58-62, Feb. 1, 1996, XP000775842, ISSN: 0722-9003), Wandt D. et al: “Continuously Tunable External Cavity Diode Laser with a Double-Grating Arrangement” (Optics Letter, Optical Society of America, Washington, US, vol. 22, no. 6, 15 Mar. 1997, pages 390-392, XP000690335, ISSN: 0146-9592), DE-A-19832750, EP-A-938171, JP-A-05 267768, U.S. Pat. No. 5,319,668, or in the co-pending European Patent applications 01121408.7, 01113372.5, or 01113371.7 of the same applicant.